Spanning tree in fabric switches

ABSTRACT

One embodiment of the present invention provides a switch. The switch includes a packet processor and a spanning tree management module. The packet processor obtains information associated with a spanning tree from a message. The spanning tree management module, in response to the obtained information being superior to locally available information of the spanning tree, determines the port role of a local port of the switch for the spanning tree to be the root port and the port state of the local port for the spanning tree to be blocking.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No. 61/771,723, Attorney Docket No. BRCD-3200.0.1.US.PSP, titled “Facilitating Spanning Tree Protocol in a Fabric Switch,” by inventors Ganesh D. Venkata, Amit Gupta, Prabu Thayalan, Vardarajan Venkatesh, Mythilikanth Raman, and Selvam Muthiah, filed 1 Mar. 2013, the disclosure of which is incorporated by reference herein.

The present disclosure is related to U.S. patent application Ser. No. 13/087,239 (Attorney Docket No. BRCD-3008.1.US.NP), titled “Virtual Cluster Switching,” by inventors Suresh Vobbilisetty and Dilip Chatwani, filed 14 Apr. 2011, and U.S. patent application Ser. No. 12/725,249 (Attorney Docket No. BRCD-112-0439US), titled “Redundant Host Connection in a Routed Network,” by inventors Somesh Gupta, Anoop Ghanwani, Phanidhar Koganti, and Shunjia Yu, filed 16 Mar. 2010, the disclosures of which are incorporated by reference herein.

BACKGROUND

1. Field

The present disclosure relates to network management. More specifically, the present disclosure relates to a method and system for efficiently facilitating external spanning tree support for a fabric switch.

2. Related Art

The growth of the Internet has brought with it an increasing demand for bandwidth. As a result, equipment vendors race to build larger and faster switches, each capable of supporting a large number of end devices, to move more traffic efficiently. However, the size of a switch cannot grow infinitely. It is limited by physical space, power consumption, and design complexity, to name a few factors. One way to meet this challenge is to interconnect a number of switches to support a large number of users. Interconnecting such a large number of switches in a layer-3 network requires tedious and complex configurations on a respective switch, typically performed by a network administrator. Such configuration includes assigning an address for a respective interface (e.g., a port) and configuring routing protocols for the switch. These issues can be solved by interconnecting switches in layer-2.

One way to increase the throughput of a switch system is to use switch stacking. In switch stacking, multiple smaller-scale, identical switches are interconnected in a special pattern to form a larger logical switch. The amount of required manual configuration and topological limitations for switch stacking becomes prohibitively tedious when the stack reaches a certain size, which precludes switch stacking from being a practical option in building a large-scale switching system.

As layer-2 (e.g., Ethernet) switching technologies continue to evolve, more routing-like functionalities, which have traditionally been the characteristics of layer-3 (e.g., Internet Protocol or IP) networks, are migrating into layer-2. Notably, the recent development of the Transparent Interconnection of Lots of Links (TRILL) protocol allows Ethernet switches to function more like routing devices. TRILL overcomes the inherent inefficiency of the conventional spanning tree protocol, which forces layer-2 switches to be coupled in a logical spanning-tree topology to avoid looping. TRILL allows routing bridges (RBridges) to be coupled in an arbitrary topology without the risk of looping by implementing routing functions in switches and including a hop count in the TRILL header.

While coupling switches in an arbitrary topology brings many desirable features to a network, some issues remain unsolved for facilitating external spanning tree support.

SUMMARY

One embodiment of the present invention provides a switch. The switch includes a packet processor and a spanning tree management module. The packet processor obtains information associated with a spanning tree from a message. The spanning tree management module, in response to the obtained information being superior to locally available information of the spanning tree, determines the port role of a local port of the switch for the spanning tree to be the root port and the port state of the local port for the spanning tree to be blocking.

In a variation on this embodiment, the switch includes a notification module which generates a notification message for a remote switch comprising the obtained information.

In a variation on this embodiment, the spanning tree management module changes the port state of the local port for the spanning tree to be forwarding in response to an approval from a remote switch.

In a variation on this embodiment, if the switch receives superior information of the spanning tree from a message from a remote switch, the spanning tree management module re-determines the port role of the local port for the spanning tree. It should be noted that the message is not a control message of the spanning tree.

In a variation on this embodiment, the spanning tree management module stores the best locally available information of the spanning tree in a local node root priority vector and the best information of the spanning tree associated with a remote switch in a local node root priority table.

In a variation on this embodiment, the switch also includes a fabric switch management module which maintains a membership in a fabric switch. The fabric switch is configured to accommodate a plurality of switches and operates as a single switch. The spanning tree management module represents the fabric switch as a single switch in the spanning tree.

In a further variation, the switch also includes a port management module which assigns a port identifier to the local port. This port identifier is unique among the edge ports of the fabric switch. The spanning tree management module uses the port identifier to participate in the spanning tree, thereby facilitating the local port to appear as a port of the single switch represented by the fabric switch.

In a further variation, the port management module stores the port identifier in an entry of a port identifier allocation table of the switch. If a status update event occurs for the switch, the port management module marks the entry as stale. Other member switches of the fabric switch are precluded from assigning this stale entry to an edge port.

In a further variation, if the effect of the status update event ends, the port management module reassigns the port identifier to the local port.

In a further variation, if a limit of the number of port identifiers has been reached and the stale entry meets one or more reallocation criteria, the port management module reassigns the port identifier of the stale entry to a second port.

In a further variation, the reallocation criteria include: (i) an age of a stale entry, and (ii) an indication whether an entry is stale due to a configured event or a learned event.

In a further variation, if a limit of the number of port identifiers has been reached and a stale entry in the port identifier allocation table is unavailable, the port management module precludes the switch from enabling the spanning tree for a local port.

In a further variation, the fabric switch management module also determines a first switch identifier for the fabric switch. This first switch identifier is distinct from a second switch identifier associated with a second fabric switch. This distinction between the first and the second switch identifiers is based on a random number or a configured number

In a variation on this embodiment, the switch also includes a link aggregation module which operates the local port in conjunction with a second port of a remote switch as a single logical port of a virtual link aggregation. The link aggregation module also selects a master switch between the switch and the remote switch. The master switch includes a selected port between the local port and the second port. This selected port actively participates in the spanning tree as representative of the logical port.

In a further variation, the link aggregation module selects the master switch based on whether a switch has received the most recent control message of the spanning tree.

In a further variation, the link aggregation module selects the master switch further based on whether a first identifier associated with the switch and the remote switch is inferior to a second identifier. The first identifier is associated with a first fabric switch and the second identifier is associated with a second fabric switch. A fabric switch is configured to accommodate a plurality of switches and operates as a single switch

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1A illustrates an exemplary fabric switch with distributed spanning tree port state determination, in accordance with an embodiment of the present invention.

FIG. 1B illustrates an exemplary node root priority vector data structure for facilitating distributed spanning tree port state determination, in accordance with an embodiment of the present invention.

FIG. 1C illustrates an exemplary node root priority table for facilitating distributed spanning tree port state determination, in accordance with an embodiment of the present invention.

FIG. 2A presents a flowchart illustrating the process of a member switch of a fabric switch determining the spanning tree port state of a local edge port, in accordance with an embodiment of the present invention.

FIG. 2B presents a flowchart illustrating the process of a member switch of a fabric switch generating a response message for determining the spanning tree port state of a remote edge port, in accordance with an embodiment of the present invention.

FIG. 3A illustrates an exemplary retentive port identifier assignment of an edge port of a fabric switch, in accordance with an embodiment of the present invention.

FIG. 3B illustrates an exemplary port identifier allocation table for retentive port identifier assignment in a fabric switch, in accordance with an embodiment of the present invention.

FIG. 4A presents a flowchart illustrating the process of a member switch of a fabric switch updating the status of port identifiers in a port identifier allocation table, in accordance with an embodiment of the present invention.

FIG. 4B presents a flowchart illustrating the process of a member switch of a fabric switch synchronizing port identifier allocation information with a newly joined member switch, in accordance with an embodiment of the present invention.

FIG. 4C presents a flowchart illustrating the process of a returning member switch of a fabric switch allocating a port identifier and assigning port state to local edge ports, in accordance with an embodiment of the present invention.

FIG. 4D presents a flowchart illustrating the process of a newly joined member switch of a fabric switch allocating a port identifier and assigning port state to local edge ports, in accordance with an embodiment of the present invention.

FIG. 5 presents a flowchart illustrating the process of a member switch of a fabric switch checking reallocation criteria for entries in a port identifier allocation table, in accordance with an embodiment of the present invention.

FIG. 6A illustrates an exemplary retentive and unique identifier assignment to fabric switches, in accordance with an embodiment of the present invention.

FIG. 6B illustrates exemplary unique identifiers for fabric switches, in accordance with an embodiment of the present invention.

FIG. 7A presents a flowchart illustrating the process of a member switch of a fabric switch associating a unique identifier with the fabric switch, in accordance with an embodiment of the present invention.

FIG. 7B presents a flowchart illustrating the process of a returning member switch of a fabric switch associating a unique identifier with the local switch, in accordance with an embodiment of the present invention.

FIG. 8A illustrates an exemplary virtual link aggregation with spanning tree support, in accordance with an embodiment of the present invention.

FIG. 8B illustrates an exemplary virtual link aggregation with spanning tree support between fabric switches, in accordance with an embodiment of the present invention.

FIG. 9 illustrates an exemplary state diagram of a partner switch of a virtual link aggregation with spanning tree support, in accordance with an embodiment of the present invention.

FIG. 10 illustrates an exemplary architecture of a switch with distributed spanning tree protocol support, in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION

The following description is presented to enable any person skilled in the art to make and use the invention, and is provided in the context of a particular application and its requirements. Various modifications to the disclosed embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the present invention. Thus, the present invention is not limited to the embodiments shown, but is to be accorded the widest scope consistent with the claims.

Overview

In embodiments of the present invention, the problem of a fabric switch participating in an external spanning tree as a single switch is solved by running a distributed spanning tree protocol for the edge ports in a respective member switch of the fabric switch. In a fabric switch, any number of switches coupled in an arbitrary topology may logically operate as a single switch. The fabric switch can be an Ethernet fabric switch or a virtual cluster switch (VCS), which can operate as a single Ethernet switch. Any member switch may join or leave the fabric switch in “plug-and-play” mode without any manual configuration. In some embodiments, a respective switch in the fabric switch is a Transparent Interconnection of Lots of Links (TRILL) routing bridge (RBridge).

When three or more switches and devices are coupled in a layer-2 network (e.g., Ethernet), there can be a layer-2 external loop (i.e., a loop created by external connectivity of a switch) among these switches. To break this external loop, a respective switch runs a spanning tree protocol, which determines the port state (e.g., blocking or forwarding) of a respective port of the switch. The forwarding ports actively participate in the spanning tree while the blocking ports do not forward packets via the spanning tree.

However, the spanning tree protocol is typically designed for individual switches locally deciding the port state. Because the fabric switch externally appears as a single switch, the fabric switch participates in the spanning tree, which is external to the fabric switch, as a single switch. Consequently, the member switches cannot make local decisions, and may need extensive and delay-prone synchronization to decide the port state of the edge ports. Other challenges include spanning tree re-convergence without topology change due to the return of the member switch to the fabric switch, identifier conflict among neighboring fabric switches, and inefficient forwarding via the edge ports in a virtual link aggregation (VLAG).

To solve this problem of a fabric switch participating in an external spanning tree as a single switch, a distributed spanning tree protocol for the edge ports in a respective member switch of the fabric switch determines the port state of the edge ports. This distributed spanning tree protocol facilitates synchronized spanning tree port state identification of the edge ports of a respective member switch of the fabric switch. The efficiency of the distributed spanning tree is further enhanced by assigning respective retentive port identifiers to the edge ports. When a member switch leaves and returns to a fabric switch, the corresponding edge ports retain the same respective port identifiers and do not cause the spanning tree to re-converge.

Furthermore, a unique layer-2 identifier is assigned to a respective fabric switch. As a result, when a plurality of fabric switches are coupled to each other, a respective fabric switch can participate in the external spanning tree protocol as a respective single switch without causing a conflict. Moreover, a plurality of the edge ports, which are in different member switches (these member switches are referred to as partner switches), can participate in a virtual link aggregation. The edge port which receives the most recent control packet is selected to participate in the external spanning tree; thus the port which provides the most efficient forwarding via the spanning is selected.

It should be noted that a fabric switch is not the same as conventional switch stacking. In switch stacking, multiple switches are interconnected at a common location (often within the same rack), based on a particular topology, and manually configured in a particular way. These stacked switches typically share a common address, e.g., an IP address, so they can be addressed as a single switch externally. Furthermore, switch stacking requires a significant amount of manual configuration of the ports and inter-switch links. The need for manual configuration prohibits switch stacking from being a viable option in building a large-scale switching system. The topology restriction imposed by switch stacking also limits the number of switches that can be stacked. This is because it is very difficult, if not impossible, to design a stack topology that allows the overall switch bandwidth to scale adequately with the number of switch units.

In contrast, a fabric switch can include an arbitrary number of switches with individual addresses, can be based on an arbitrary topology, and does not require extensive manual configuration. The switches can reside in the same location, or be distributed over different locations. These features overcome the inherent limitations of switch stacking and make it possible to build a large “switch farm,” which can be treated as a single, logical switch. Due to the automatic configuration capabilities of the fabric switch, an individual physical switch can dynamically join or leave the fabric switch without disrupting services to the rest of the network.

Furthermore, the automatic and dynamic configurability of the fabric switch allows a network operator to build its switching system in a distributed and “pay-as-you-grow” fashion without sacrificing scalability. The fabric switch's ability to respond to changing network conditions makes it an ideal solution in a virtual computing environment, where network loads often change with time.

Although the present disclosure is presented using examples based on the layer-2 communication protocol, embodiments of the present invention are not limited to layer-2 networks. Embodiments of the present invention are relevant to any networking protocol which requires a loop-free network topology. In this disclosure, the term “layer-2 network” is used in a generic sense, and can refer to any networking layer, sub-layer, or a combination of networking layers below layer-3 (e.g., the network layer in the Internet protocol stack).

The term “RBridge” refers to routing bridges, which are bridges implementing the TRILL protocol as described in IETF Request for Comments (RFC) “Routing Bridges (RBridges): Base Protocol Specification,” available at http://tools.ietf.org/html/rfc6325, which is incorporated by reference herein. Embodiments of the present invention are not limited to application among RBridges. Other types of switches, routers, and forwarders can also be used.

In this disclosure, the term “end device” can refer to a host machine, a conventional layer-2 switch, or any other type of network device. Additionally, an end device can be coupled to other switches or hosts further away from a layer-2 network. An end device can also be an aggregation point for a number of network devices to enter the layer-2 network.

The term “edge port” refers to a port on a fabric switch which exchanges data frames with a network device outside of the fabric switch (i.e., an edge port is not used for exchanging data frames with another member switch of a fabric switch). In a generic sense, the term “port” can refer to any interface of a switch, including an “edge port.” The term “inter-switch port” refers to a port which sends/receives data frames among member switches of a fabric switch. The terms “interface” and “port” are used interchangeably.

The term “switch identifier” refers to a group of bits that can be used to identify a switch. Examples of a switch identifier include, but are not limited to, a MAC address, an Internet Protocol (IP) address, and an RBridge identifier. Note that the TRILL standard uses “RBridge ID” to denote a 48-bit intermediate-system-to-intermediate-system (IS-IS) System ID assigned to an RBridge, and “RBridge nickname” to denote a 16-bit value that serves as an abbreviation for the “RBridge ID.” In this disclosure, “switch identifier” is used as a generic term, is not limited to any bit format, and can refer to any format that can identify a switch. The term “RBridge identifier” is also used in a generic sense, is not limited to any bit format, and can refer to “RBridge ID,” “RBridge nickname,” or any other format that can identify an RBridge.

The term “frame” refers to a group of bits that can be transported together across a network. “Frame” should not be interpreted as limiting embodiments of the present invention to layer-2 networks. “Frame” can be replaced by other terminologies referring to a group of bits, such as “message,” “packet,” “cell,” or “datagram.”

The term “loop” is used in a generic sense, and it can refer to any number of standalone and fabric switches coupled to each other in such a way that at least one of the switches may receive a frame previously originated from the same switch. The term “external loop” refers to a network loop formed based on the external connectivity of a switch. For a fabric switch, an external loop is formed by the edge interfaces. The terms “external loop” and “loop” are used interchangeably in a generic sense. The term “loop breaking” refers to disabling an interface or a link between two switches belonging to a loop in so that the loop does not exist any longer.

The term “spanning tree protocol” is used in a generic sense, and can refer to any protocol that creates a spanning tree in a network. Such a protocol can be distributed or centralized. Examples of such protocols include, but are not limited to, Spanning Tree Protocol (STP), Rapid Spanning Tree Protocol (RSTP) and Multiple Spanning Tree Protocol (MSTP). The term “spanning tree” is also used in a generic sense, and can refer to any loop-free topology in a network.

The term “fabric switch” refers to a number of interconnected physical switches which form a single, scalable logical switch. In a fabric switch, any number of switches can be connected in an arbitrary topology and the entire group of switches functions together as one single switch. This feature makes it possible to use many smaller, inexpensive switches to construct a large fabric switch, which can be viewed externally as a single switch.

Network Architecture

FIG. 1A illustrates an exemplary fabric switch with distributed spanning tree port state determination, in accordance with an embodiment of the present invention. As illustrated in FIG. 1A, a layer-2 network 100 (e.g. Ethernet) includes fabric switch 102, and switches 104, 106, and 108. Fabric switch 102 includes member switches 112, 114, 116, and 118. In some embodiments, one or more switches, including one or more member switches of fabric switch 102, in network 100 can be virtual switches (e.g., a software switch running on a computing device).

In some embodiments, fabric switch 102 is a TRILL network and a respective member switch of fabric switch 102, such as switch 116, is a TRILL RBridge. Switches in fabric switch 102 use edge ports to communicate with end devices (e.g., non-member switches) and inter-switch ports to communicate with other member switches. For example, switch 116 is coupled to switch 106 via an edge port and to switches 112, 114, and 118 via inter-switch ports and one or more links. Data communication via an edge port can be based on Ethernet and via an inter-switch port can be based on TRILL protocol. It should be noted that control message exchange via inter-switch ports can be based on a different protocol (e.g., Internet Protocol (IP) or Fibre Channel (FC) protocol).

In network 100, member switches 116 and 118 of fabric switch 102 are coupled to switches 106 and 108, respectively. Switches 106 and 108 are further coupled to switch 104 and form a loop in network 100. This loop is external to fabric switch 102 and may not be relevant to internal communication of fabric switch 102 (e.g., among the member switches). To break the loop, switches 102, 104, 106, and 108 participate in a spanning tree. Fabric switch 102 operates as a single switch and appears as a single switch to switches 104, 106, and 108. Hence, fabric switch 102 participates in the spanning tree protocol as a single switch.

A respective edge port for which spanning tree has been enabled can participate in the spanning tree protocol. A respective member switch of fabric switch 102 runs a distributed spanning tree algorithm for these edge ports. To facilitate the operations of the distributed spanning tree algorithm, a respective edge port of fabric switch 102 for which spanning tree is enabled is assigned a port identifier. This port identifier is unique across fabric switch 102. In this way, a respective port is uniquely identified to external end devices. For example, port 122 of switch 116 and port 124 of switch 118 may have the same local identifier but the fabric-wide port identifiers for ports 122 and 124 are unique. As a result, switch 106 or 108 can uniquely identify the port of fabric switch 102 with which switch 106 or 108 is communicating.

Suppose that switch 104 is the root switch for the spanning tree. During operation, switches 106 and 108 receive a proposal message comprising spanning tree information, such as the path cost to root switch 104. If the information received is superior to the information available at switches 106 and 108 (e.g., has a lower path cost to root 104 and/or a designated switch identifier with a lower value), these switches respond via respective receiving ports with corresponding agreement messages. These ports become the root ports of switches 106 and 108 for the spanning tree. In other words, the port role assigned to these ports is root port. In some embodiments, the proposal and agreement messages are respective Bridge Protocol Data Units (BPDUs). A BPDU can include the root switch identifier, the path cost from the sender switch to the root switch, an identifier of the sender switch, and an identifier of the port via which the BPDU has been sent.

However, when switches 106 and 108 select the port role to be root port, switches 106 and 108 send proposal messages to downstream switches via downstream ports 126 and 128, respectively, and put all other ports (ports other than the root port and the downstream port) in a blocking state. Examples of a blocking state include, but are not limited to, a blocking state when a port does not send or receive any user data, a listening state when a port receives BPDUs, but does not learn media access control (MAC) addresses or forward data, and a learning state when a port learns MAC addresses but does not forward data. If switch 106 does not receive an agreement message back via port 126, switch 106 can put port 126 in a blocking state as well. Similarly, if switch 108 does not receive an agreement message back via port 128, switch 108 can put port 128 in a blocking state.

Switch 106 sends a proposal message 132 comprising the path cost to root switch 104 to member switch 116. Switch 116 receives proposal message 132 via port 122. Similarly, switch 108 sends a proposal message 134 comprising the path cost to root switch 104 to member switch 118. Switch 118 receives proposal message 134 via port 124. Without running the distributed spanning tree algorithm, if the received information is superior to the information available at switches 116 and 118, these switches respond via ports 122 and 124, respectively, with corresponding agreement messages. Then both ports 122 and 124 become root ports. In other words, the port role assigned to ports 122 and 124 is root port.

However, because switches 116 and 118 are member switches of fabric switch 102, which participates in the spanning tree as a single switch, if both ports 122 and 124 become root ports, the loop persists. On the other hand, if switch 116 or 118 exchanges information via internal messaging with all other member switches to determine whether port 122 or 124, respectively, is the superior port in the entire fabric switch 102, there can be a delay due to the internal messaging. Meanwhile, because switches 106 and 108 may run a standard spanning tree protocol, switches 106 and 108 expect agreement messages via ports 126 and 128, respectively. Due to the delay, switches 106 and 108 may time out proposal messages 132 and 134, and put ports 126 and 128, respectively, in a blocking state.

To solve this problem, a respective member switch of fabric switch 102 runs the distributed spanning tree protocol for the spanning-tree-enabled edge ports of fabric switch 102. This algorithm performs an extra validation for a port transitioning to a root port (i.e., a port being assigned the role of a root port). It should be noted that this validation is for transitioning to a root port and is done if the current port state of the port in consideration is not forwarding. For example, when member switch 116 transitions port 122 to a root port (e.g., changes the spanning tree state machine associated with port 122), or responds to proposal message 132, switch 116 compares the information in the proposal message with the information available locally at switch 116. This information can include available port information associated with one or more port identifiers of fabric switch 102 (e.g., information associated with the port in other member switches). If switch 116 determines that proposal message 132 includes superior information, switch 116 sends an agreement message 142 to switch 106 and transitions port 122 to a root port. However, switch 116 does not set the port state to forwarding. This port state can be referred to as a “root/blocking” state, distinct from a regular root port, which is in a forwarding state by default.

Switch 116 then sends a notification message for switches 112, 114, and 118, comprising information associated with the updated port state. In some embodiments, the notification message is in an internal messaging format for fabric switch 102. Examples of an internal messaging format include, but are not limited to, FC, Ethernet, and TRILL. If port 122 has the most suitable path (e.g., the least-cost path) to root switch 104, switches 112, 114, and 118 send respective response messages to switch 116. A response message can include an approval for port 122 to be the root port, or local superior information if the member switch has a more suitable path to root switch 104 than via port 122. For example, switch 118 can have superior information in proposal message 134 and includes that information in the response message to switch 116. Switch 116 transitions the port state of port 122 to a forwarding state if switch 116 receives approvals from switches 112, 114, and 118. This port state can be referred to as a “root/forwarding” state. Otherwise, switch 116 reselects the port state and/or the port role for port 122, and transitions port 122 to the selected port state and/or the port role.

Similarly, upon receiving proposal message 134, switch 118 determines whether proposal message 134 includes superior information compared to the information available locally at switch 118. If so, switch 118 sends agreement message 144 to switch 108, puts port 124 in a root/blocking state, and sends a notification message to switches 112, 114, and 116. Switches 112, 114, and 116 send respective response messages to switch 118. Switch 118 transitions the port state of port 124 to a root/forwarding state if switch 118 receives approvals from switches 112, 114, and 116. Otherwise, switch 118 reselects the port state and/or the port role for port 124, and transitions port 124 to the selected port state and/or the port role.

Upon receiving agreement messages 142 and 144, respectively, switches 106 and 108 consider ports 126 and 128, respectively, to be designated ports for the spanning tree. Hence, switches 106 and 108 start forwarding frames via ports 126 and 128, respectively, toward fabric switch 102. However, when ports 122 and 124 are in root/blocking state, these frames are discarded at switches 116 and 118, respectively. As a result, the loop is broken in network 100. When one of ports 122 and 124 transitions to a root/forwarding state, that port starts processing received frames. In this way, the distributed spanning tree protocol prevents external loops for a fabric switch while conforming to standard spanning tree protocols. It should be noted that fabric switch 102, as a single switch, is associated with a switch identifier (e.g., a MAC address), which represents fabric switch 102 as a single switch. A respective member switch of fabric switch 102 is associated with that identifier. Switches 116 and 118 use that identifier as the switch identifier in agreement messages 142 and 144, respectively.

Root Priority Vectors

The distributed spanning tree algorithm runs individual spanning tree state machine at a respective member switch of fabric switch 102. At the same time, a respective member switch considers spanning tree information from other member switches in fabric switch 102 for the most suitable information for the entire fabric. To ensure this, a respective member switch in fabric switch 102 maintains a node root priority vector (NRPV), which represents the best spanning tree information that is received via a local port of the member switch. In other words the node root priority vector stores the best locally available information for the spanning tree. For example, the node root priority vector of switch 116 can be a data structure which stores the spanning tree information associated with port 122, which can become the root port for fabric switch 102 if port 122's information is superior compared with other edge ports of fabric switch 102. Switch 116 calculates its own node root priority vector and stores the best locally available information for the spanning tree at switch 116. Switch 116 then generates a notification message comprising the node root priority vector and sends the notification message to all other member switches of fabric switch 102.

Similarly, switch 118 calculates the node root priority vector for port 124, generates a notification message comprising the node root priority vector, and sends the notification message to all other member switches of fabric switch 102. In some embodiments, switches 112 and 114 also calculate their respective node root priority vector for the spanning tree even though these switches do not have an edge port participating in the spanning tree. Whenever a member switch receives better information (e.g., a lower-cost path) via any of the ports of the switch, or the port associated with the current node root priority vector becomes unavailable or starts receiving inferior information, the member switch recalculates the node root priority vector.

Since the member switches of fabric switch 102 exchange their respective node root priority vector with each other, the member switches can use this information to calculate the best spanning tree information received within fabric switch 102. This best information can be referred to as the cluster root priority vector (CRPV). The cluster root priority vector includes the spanning tree information which represents the root port for entire fabric switch 102. For example, switch 116's node root priority vector can represent port 122 and switch 118's node root priority vector can represent port 124. However, if port 122 is the root port for fabric switch 102, the cluster root priority vector represents port 122.

FIG. 1B illustrates an exemplary node root priority vector data structure for facilitating distributed spanning tree port state determination, in accordance with an embodiment of the present invention. In this example, a node root priority vector data structure 150 includes a switch role 152, which indicates the type of role of a member switch. For example, if the switch has a root port, switch role 152 indicates that the switch is a designated switch. The node root priority vector also includes a designated bridge priority vector 154. In some embodiments, designated bridge priority vector 154 is the best of the root priority vectors received via the edge ports of the switch. The root priority vector represents the information included in a BPDU (e.g., a proposal message) received via an edge port. In some embodiments, node root priority vector data structure 150 is maintained per spanning tree instance at a member switch.

Designated bridge priority vector 154 includes root switch identifier 156 (identifier of the root switch of the spanning tree instance), root path cost 158 (the path cost between the member switch and the root switch), designated switch identifier 160 (identifier of the designated switch from which the member switch has received a proposal message), designated port identifier 162 (identifier of the designated port of the designated switch from which the member switch has received a proposal message), and port identifier 164 (the unique port identifier for the fabric switch assigned to the edge port of the member switch via which the proposal message has been received). If the switch role is the root switch, designated bridge priority vector 154 is not applicable. In some embodiments, node root priority vector data structure 150 can further include other information, such as a hello timer, a forward delay timer, a maximum age of a vector, and a message age.

FIG. 1C illustrates an exemplary node root priority table for facilitating distributed spanning tree port state determination, in accordance with an embodiment of the present invention. In this example, a node root priority table 170 includes node root priority vector information for fabric switch 102, as described in conjunction with FIG. 1A. Suppose that switch 116 is the designated switch for fabric switch 102, and entries 172 and 176 correspond to node root priority vectors of switches 116 and 118, respectively. Entry 172 includes a switch identifier 182 and node root priority vector 174 of switch 116. Similarly, entry 176 includes a switch identifier 184 and node root priority vector 178 of switch 118.

Node root priority vector 174 indicates the switch role to be designated, and includes the switch identifier of root switch 104; the root path cost to switch 116 (the path cost between root switch 104 and switch 116); the switch identifier of designated switch 106, which sends proposal message 132 to switch 116; the port identifier of designated port 126, via which proposal message 132 is sent; and port identifier 192 of port 122, via which proposal message 132 is received. Port identifier 192 is the unique port identifier for fabric switch 102 assigned to port 122. Similarly, node root priority vector 178 indicates switch role to be blocked, and includes the switch identifier of root switch 104; the root path cost to switch 118; the switch identifier of designated switch 108 which sends proposal message 134 to switch 118; the port identifier of designated port 128, via which proposal message 134 is sent; and port identifier 194 of port 124, via which proposal message 134 is received. Port identifier 194 is the unique port identifier for fabric switch 102 assigned to port 124.

Even through FIG. 1C illustrates entries for switches 116 and 118, node root priority table 170 can also include entries for switches 112 and 114. A respective member switch in fabric switch 102 maintains a node root priority table. The member switch can use the information in the node root priority table to calculate the cluster root priority vector for fabric switch 102. In the example in FIG. 1C, node root priority vector 174 in entry 172 is the cluster root priority vector for fabric switch 102.

Port State Determination

FIG. 2A presents a flowchart illustrating the process of a member switch of a fabric switch determining the spanning tree port state of a local edge port, in accordance with an embodiment of the present invention. During operation, the switch receives a proposal via a local edge port (operation 202). The switch checks whether the proposal includes superior information (operation 204). If not, the switch can discard the proposal message. If the proposal includes superior information, the switch generates an agreement message in response to the proposal message (operation 206). The switch puts any current forwarding local port of the spanning tree (e.g., the local port which is in a forwarding state and participating in the spanning tree) in a blocking state (operation 208). The switch transmits the agreement message via the edge port and puts that edge port in a root/blocking state (operation 210), as described in conjunction with FIG. 1A.

The switch then generates a notification message for other member switches of the fabric switch comprising the local port information (operation 212). In some embodiments, this local port information is represented by a node root priority vector, as described in conjunction with FIG. 1B. The switch determines respective output ports corresponding to respective member switches for the generated notification message and forwards the notification message via the determined output ports (operation 214). The switch receives response messages from other member switches in response to the notification message (operation 216). This response message can include an approval or superior information from a remote edge port (an edge port in a different member switch) of the fabric switch.

The switch checks whether the switch has received approval from the other member switches (operation 218). If the switch has received approval from all other member switches, the local edge port has the superior information for the entire fabric switch for the spanning tree. The switch then sets the edge port in a root/forwarding state (operation 220). If the switch has not received approval from all other member switches, a remote edge port has the superior information for the entire fabric switch for the spanning tree. The switch then reselects the port state for the edge port based on the received response messages and sets the edge port in the selected port state (operation 222). In some embodiments, the response message is based on the internal messaging of the fabric switch and is not a control message of the spanning tree.

FIG. 2B presents a flowchart illustrating the process of a member switch of a fabric switch generating a response message for determining the spanning tree port state of a remote edge port, in accordance with an embodiment of the present invention. During operation, the switch receives a notification message via an inter-switch port (operation 252). The switch extracts port information associated with a remote edge port from the notification message (operation 254) and compares the extracted information with locally available information (operation 256). In some embodiments, the switch obtains the locally available information from a local node root priority table, as described in conjunction with FIG. 1C. The switch checks whether the received information is superior (operation 258). If so, the switch generates a response message comprising an approval (operation 262). Otherwise, the switch generates a response message comprising superior locally available information (operation 264). The switch then determines an output port for the generated response message and sends the response message via the determined output port (operation 266).

Retentive Port Identifier

Each port of a switch participating in a spanning tree should be assigned a unique identifier. With existing technologies, this port identifier in a typical switch is the physical port number. However, in a fabric switch, the same physical port number can be associated with edge ports of different member switches and cause a conflict. A fabric-switch-wide unique port identifier resolves this conflict and uniquely identifies a respective edge port of a fabric switch in a spanning tree. In the example in FIG. 1A, if port 122 of switch 116 in fabric switch 102 is a designated port of a spanning tree, other switches in network 100 can uniquely identify port 122 using its port identifier 192, as described in conjunction with FIG. 1C. This allows fabric switch 102 to participate in a spanning tree as a single switch.

However, for spanning tree protocols, such as RSTP and MSTP, a port identifier can be 16 bits long with two parts. The first part is 4 bits long and indicates a port priority. The second part is 12 bits long and indicates a port number. For STP, the second part is 8 bits long. As a result, the maximum number of port numbers associated with a switch becomes restricted. However, because a fabric switch comprises a plurality of physical switches, the number of ports of a fabric switch can be significantly large and 8 bits may not be enough to represent each of these ports. Furthermore, if a member switch leaves and reenters a fabric switch, the edge ports of that member switch can receive new port identifiers. As a result, even though the topology has not been changed, the spanning tree may re-converge, causing inefficiency in the network. Embodiments of the present invention solve this problem by allowing an edge port to retain its fabric-switch-wide unique port identifier, using identifiers only for spanning-tree-enabled edge ports, and reusing stale identifiers for new edge ports.

FIG. 3A illustrates an exemplary retentive port identifier assignment of an edge port of a fabric switch, in accordance with an embodiment of the present invention. In this example, unique port identifiers 192 and 194 are assigned to ports 122 and 124, respectively, as described in conjunction with FIG. 1C. Port identifiers 192 and 194 are assigned when spanning tree is enabled for ports 122 and 124, respectively. In some embodiments, port identifiers for fabric switch 102 are assigned by a computer system, such as an administrator device (not shown) or a designated member switch, and stored in a port number allocation table. Port identifiers 192 and 194 remain tied to ports 122 and 124, respectively, until fabric switch 102 runs out of available port identifiers for fabric switch 102. In some embodiments, the number of available port identifiers for fabric switch 102 is determined based on the spanning tree protocol fabric switch 102 is participating in.

Suppose that a status update event 310 occurs for member switch 116. A status update event can change the status of a port's entry in the port number allocation table. Status update event 310 can be a learned event, which a member switch of a fabric switch learns, or a configured event, which is configured for a member switch. Examples of a learned state update event include, but are not limited to, a switch failure, a line card failure, and a link failure. Examples of a configured event include, but are not limited to, spanning tree disablement for an edge port and switching off a member switch. In the example in FIG. 3A, if port 122 becomes unavailable due to a status update event (e.g., switch 116 fails or becomes disconnected in fabric switch 102), port 122 no longer participates in the spanning tree. However, port 122's port identifier 192 is not reused for other edge ports of fabric switch 102. Instead, port 122's entry in the port number allocation table of fabric switch 102 is marked as “stale.”

During operation, a new member switch 312 joins fabric switch 102 as a member switch (denoted with dotted lines). Switch identifier 382 is then assigned to switch 312. This assignment can be automatic, without any manual configuration, or based on the configuration of a user (e.g., a network administrator). Any edge port of switch 312 is assigned a new port identifier instead of stale port identifier 192. In some further embodiments, newly joined member switch 312 receives the most recent port identifier allocation table from other member switches of fabric switch 102, allocates the port identifier to local edge port 322, updates the local port identifier allocation table accordingly, and synchronizes the updated port identifier allocation table with other member switches.

It should be noted that multiple member switches can concurrently try to allocate port identifiers to local edge ports. Suppose that when switch 312 is joining fabric switch 102, spanning tree is enabled for edge port 324 of switch 112. This edge port can couple switch 304 with switch 112. As a result, switches 312 and 112 can concurrently try to assign a port identifier to ports 322 and 324, respectively. This can lead to a race condition. This problem is solved by associating a fabric-wide lock with a respective port identifier assignment. For example, when switch 312 is allocating a port identifier for port 322, switch 312 obtains the lock and notifies all other member switches. As a result, other member switches refrain from allocating an identifier to any local edge port. Switch 312 allocates a port identifier to local edge port 322, updates the local port number allocation table accordingly, synchronizes the updated port number allocation table with other member switches, and releases the lock. Switch 112 then obtains the lock, allocates the next available port identifier to local edge port 324, updates the local port number allocation table accordingly, synchronizes the updated port number allocation table with other member switches, and releases the lock.

In the example in FIG. 3A, edge port 322 of member switch 312 is coupled to switch 302 and participates in the spanning tree. A new fabric-switch-wide unique port identifier 392 is assigned to port 322. However, when spanning tree is enabled for port 322 and no new unique port number is available, stale port identifier 192 can be assigned to port 322. Otherwise, port identifier 192 remains persistently tied to port 122. When the effect of status update event 310 ends (e.g., switch 116 recovers from failure or becomes reconnected in fabric switch 102), the same port identifier 192 is assigned to port 122. This prevents topology changes due to changes of the port identifier of a port. In some embodiments, the port identifier allocation table is synchronized with a respective member switch of fabric switch 102 to ensure that the port identifier allocation information is not lost if one or more member switches leave fabric switch 102.

FIG. 3B illustrates an exemplary port identifier allocation table for retentive port identifier assignment in a fabric switch, in accordance with an embodiment of the present invention. In this example, port identifier allocation table 350 represents port identifier allocation to the edge ports of fabric switch 102 following status update event 310 and enabling of the spanning tree for port 322. A respective entry of table 350 includes a port identifier 352, an interface name 354, and a status 356 for the entry. Entry 362 of table 350 includes port identifier 192, which is allocated to port 122; an interface name of port 122; and a status indicating that entry 362 is stale. In some embodiments, interface name of a port is based on the switch identifier of the member switch, which includes the port; a line card number of the line card, which includes the port; and a physical port number. Suppose that the line card number for port 122 is 372 and the physical port number of port 122 in switch 116 is 382. Then the interface name for port 122 can be 182/372/382. In some embodiments, if a switch has only one line card (e.g., in a “pizza-box” switch type), the line card number can be zero (“0”)_(.)

Similarly, entry 364 includes port identifier 194, which is allocated to port 124; an interface name of port 124; and a status indicating that entry 364 is active (or used). If a status is active, the corresponding port identifier is not assigned to any other edge port of fabric switch 102. Suppose that the line card number for port 124 is 374 and the physical port number of port 124 in switch 118 is 384. Then the interface name for port 124 can be 184/374/384. Entry 366 includes port identifier 392, which is allocated to port 322; an interface name of port 322; and a status indicating that entry 366 is active. Suppose that the line card number for port 322 is 376 and the physical port number of port 322 in switch 312 is 386. Then the interface name for port 322 can be 184/376/386.

If spanning tree is enabled for another edge port of fabric switch 102, port identifier allocation table 350 ensures that already assigned identifiers are not reassigned to that edge port. If the maximum number of assignable port identifiers for fabric switch 102 is reached and no stale entry is available in table 350, spanning tree may not be enabled for that edge port. In this way, table 350 facilitates allocation of a unique port identifier to a respective spanning-tree-enabled edge port of fabric switch 102. By combining the switch identifier, line card number, and physical port number of a port for the interface name, a respective port of a fabric switch is uniquely identified by the interface name. Table 350 maps that interface name to the port identifier, thereby tying the port identifier to the physical port. It should be noted that an interface name by itself may not be suitable for a spanning tree because the interface name may not be compatible with a spanning tree protocol. Table 350 further facilitates persistent port number allocation for the port identifiers.

Retentive Port Identifier Processing

FIG. 4A presents a flowchart illustrating the process of a member switch of a fabric switch updating the status of port identifiers in a port identifier allocation table, in accordance with an embodiment of the present invention. During operation, the switch detects a status update event associated with a member switch, which can be the local switch or a remote switch, of the fabric switch (operation 402). The switch identifies the edge ports of the switch which have spanning tree enabled and are affected by the status update event (operation 404). The switch then marks the entries of the local port identifier allocation table comprising the port identifiers of the identified edge ports as “stale” (operation 406), as described in conjunction with FIG. 3B. The switch synchronizes the updated port identifier allocation table with other member switches (operation 408). This synchronization process can comprise an exchange of the most recent port identifier allocation table among member switches.

FIG. 4B presents a flowchart illustrating the process of a member switch of a fabric switch synchronizing port identifier allocation information with a newly joined member switch, in accordance with an embodiment of the present invention. During operation, the switch detects a newly joined member switch of the fabric switch (operation 422). This newly joined member switch can be a new switch joining the fabric switch or a returning member switch, which has left the fabric switch. The switch then generates a message comprising the current (e.g., the most recent) port identifier allocation table (operation 424) and sends the generated message to the newly joined member switch (operation 426). The switch receives a confirmation message comprising the current port identifier allocation table of the newly joined member switch (operation 428) and updates the local port identifier allocation table based on the received table (operation 430).

FIG. 4C presents a flowchart illustrating the process of a returning member switch of a fabric switch allocating a port identifier and assigning port state to local edge ports, in accordance with an embodiment of the present invention. During operation, the switch joins the fabric switch as a returning member switch and identifies local edge ports which have the spanning tree enabled (operation 452). A returning member switch is a switch for which a status update event has been completed, such as a switch recovered from a failure. The switch then retrieves the previous local port identifier allocation table (operation 454). This table can be retrieved with other previous local configurations of the switch prior to the status update event. The switch receives notification message(s) comprising the current port identifier allocation table from one or more other member switch(es) (operation 456) and updates the previous local port identifier allocation table based on the received table(s) (operation 458).

The switch checks whether the updated port identifier allocation table has stale entries for the identified ports (operation 460), as described in conjunction with FIG. 3A. If the table has stale entries for the identified ports, the switch reallocates the port identifiers of the corresponding stale entries to the identified edge ports, marks the stale entries as “active,” and notifies other member switches of the updated port identifier allocation table (operation 462). This notification process includes generating a message, identifying output ports for the message, and sending the message via the output port.

If the table does not have stale entries for the identified ports, the previously assigned port identifiers have been assigned to other edge ports of the fabric switch. The switch then checks whether the limit has been reached for port identifiers (operation 464). In some embodiments, this limit is determined based on the number of identifiers a spanning tree protocol allows for a switch. If the limit has not been reached, the switch allocates new port identifiers to the identified edge ports and updates the local port identifier allocation table accordingly (operation 466). In some embodiments, the new port identifiers are generated sequentially. If the limit has been reached, the switch checks whether any stale entry is available in the local port identifier allocation table (operation 468). If no stale entry is available, spanning tree cannot be enabled for that edge port. The switch then puts that edge port in a blocking state and notifies other member switches accordingly (operation 470).

If a stale entry is available, the switch identifies the stale entries which meet reallocation criteria (operation 472). In some embodiments, reallocation criteria include an age of a stale entry (e.g., the oldest entry is selected first for reassignment), and an indication whether an entry is stale due to a configured event or a learned event (e.g., a configured entry is selected first for reassignment). Checking reallocation criteria for entries in a port identifier allocation table is discussed in further detail in conjunction with FIG. 5. The switch then allocates the port identifiers from the identified stale entries to the identified edge ports (operation 474) and assigns the port states to identified ports in conjunction with other member switches (operation 476), as described in conjunction with FIG. 2B.

FIG. 4D presents a flowchart illustrating the process of a newly joined member switch of a fabric switch allocating a port identifier and assigning port state to local edge ports, in accordance with an embodiment of the present invention. During operation, the switch joins the fabric switch as a new member switch and identifies local edge ports which have spanning tree enabled (operation 482). The switch receives notification message(s) comprising the current port identifier allocation table from one or more other member switch(es) (operation 484). The switch then checks whether the limit has been reached for port identifiers (operation 486). If the limit has not been reached, the switch allocates new port identifiers to the identified edge ports and updates the local port identifier allocation table accordingly (operation 488).

If the limit has been reached, the switch checks whether any stale entry is available in the local port identifier allocation table (operation 490). If no stale entry is available, spanning tree cannot be enabled for that edge port. The switch then puts that edge port in a blocking state and notifies other member switches accordingly (operation 492). If a stale entry is available, the switch identifies the stale entries which meet reallocation criteria (operation 494). Checking reallocation criteria for entries in a port identifier allocation table is discussed in further detail in conjunction with FIG. 5. The switch then allocates the port identifiers from the identified stale entries to the identified edge ports (operation 496) and assigns the port states to identified ports in conjunction with other member switches (operation 498), as described in conjunction with FIG. 2B.

FIG. 5 presents a flowchart illustrating the process of a member switch of a fabric switch checking reallocation criteria for entries in a port identifier allocation table, in accordance with an embodiment of the present invention. During operation, the switch determines the number of required port identifiers to be X (operation 502) and identifies the stale entries in the local port identifier allocation table (operation 504). The switch then checks whether the port identifier allocation table has at least X stale entries for configured events (operation 506). If the port identifier allocation table has at least X stale entries for configured events, the switch identifies the X oldest stale entries for configured events (operation 508).

If the port identifier allocation table does not have at least X stale entries for configured events, the switch identifies Y available stale entries for configured events (operation 514) and identifies (X-Y) oldest stale entries for learned events (operation 516). After identifying the stale entries (operation 508 or 516), the switch determines the X identified entries to be stale entries meeting the reallocation criteria (operation 510). In some embodiments, the switch can obtain user confirmation of stale entries meeting reallocation criteria (operation 512). For example, the switch can provide a warning message to the user indicating that the port identifiers which are about to be reallocated. These port identifiers are reallocated when the user confirms the selection of the port identifiers for reallocation. If the user does not confirm the selection, the identifiers of the identified stale entries are not reassigned.

Unique Identifier for Fabric Switches

An identifier is associated with a fabric switch and identifies the fabric switch as a single switch. This identifier is associated with a respective member switch of the fabric switch. However, because a fabric switch comprises a plurality of member switches, and a member switch can be swapped from one fabric switch to another fabric switch, the identifier of the fabric switch should not be associated with any specific physical switch and should be tied to the fabric switch. Furthermore, if two neighboring fabric switches participating in a spanning tree have the same identifier, switches in the spanning tree can consider both switches as the same switch.

FIG. 6A illustrates an exemplary retentive and unique identifier assignment to fabric switches, in accordance with an embodiment of the present invention. As illustrated in FIG. 6A, a layer-2 network 600 (e.g. Ethernet) includes fabric switches 602 and 604, and switch 606. Fabric switch 602 includes member switches 612, 614, 616, and 618, and fabric switch 604 includes member switches 622, 624, 626, and 628. In some embodiments, one or more switches, including one or more member switches of fabric switches 602 and 604, in network 600 can be virtual switches (e.g., a software switch running on a computing device).

In some embodiments, fabric switches 602 and 604 are respective TRILL networks and respective member switches of fabric switches 602 and 604, such as switches 618 and 626, are TRILL RBridges. Switches in fabric switches 602 and 604 use edge ports to communicate with end devices (e.g., non-member switches) and inter-switch ports to communicate with other member switches. Data communication via an edge port can be based on Ethernet and via an inter-switch port can be based on TRILL protocol. It should be noted that control message exchange via inter-switch ports can be based on a different protocol (e.g., IP or FC protocol).

In network 600, member switches 618 and 626 of fabric switches 602 and 604, respectively, are coupled to switch 606. Switches 602, 604, and 606 participate in a spanning tree as a single switch. Fabric switches 602 and 604 operate as respective single switches and appear as a single switch to switch 606. Hence, fabric switches 602 and 604 participate in the spanning tree protocol as a single switch. During operation, switch 606 sends proposal messages 632 and 634 to switches 618 and 626, respectively. Switches 618 and 626 consider the received information to be superior to any locally available information of the spanning tree, and respond by sending agreement messages 642 and 644, respectively.

It should be noted that fabric switch 602, as a single switch, is associated with an identifier (e.g., a MAC address). This identifier is used in agreement message 642 as the switch identifier. As a result, upon receiving agreement message 642, switch 606 considers fabric switch 602 as a single switch. Similarly, fabric switch 604, as a single switch, is associated with an identifier. This identifier is used in agreement message 644 as the switch identifier. As a result, upon receiving agreement message 644, switch 606 considers fabric switch 604 as a single switch. Because different fabric switches are often configured separately, fabric switches 602 and 604 can be configured with the same fabric switch identifier. If this same fabric switch identifier is used to derive the switch identifier (e.g., a MAC address) of fabric switches 602 and 604, these two fabric switches can have the same switch identifier.

In the example of FIG. 6A, if both fabric switches 602 and 604 have the same switch identifier, switch 606 considers agreement messages 642 and 644 to be from the same switch. In response, switch 606 perceives a loop, and blocks one of the corresponding ports. As a result, one of fabric switches 602 and 604 can become disconnected. Embodiments of the present invention solve this problem by combining an organizationally unique identifier (OUI), a fabric switch identifier, and a random number or a configured number to generate a switch identifier for a fabric switch. When switches 602 and 604 are configured with the same fabric switch identifier, the random number can generate two distinct switch identifiers. Even when the random numbers of fabric switches 602 and 604 are same, a user can configure a portion of the switch identifiers to make them distinct.

FIG. 6B illustrates exemplary unique identifiers for fabric switches, in accordance with an embodiment of the present invention. In this example, switch identifier 650-A is generated based on an organizationally unique identifier 652, fabric switch identifier 654, and random number 656. Organizationally unique identifier 652 is a number that uniquely identifies a vendor, manufacturer, or other organization and is used as the first portion of derivative identifiers to uniquely identify a particular piece of equipment. For example, organizationally unique identifier 652 can be the prefix of a MAC address. Fabric switch identifier 654 is an identifier assigned to a fabric switch, and is associated with a respective member switch of the fabric switch. If switch identifier 650-A is the same as a switch identifier of another fabric switch, a configured number 658 can be used to generate switch identifier 650-B instead of random number 656. In some embodiments, random number 656 and configured number 658 are one byte long, and fabric switch identifier 654 is two bytes long.

FIG. 7A presents a flowchart illustrating the process of a member switch of a fabric switch associating a unique identifier with the fabric switch, in accordance with an embodiment of the present invention. During operation, the switch obtains an organizationally unique identifier and a fabric switch identifier of the fabric switch (operation 702) and generates a random number (operation 704). The switch then generates a switch identifier for the fabric switch based on the organizationally unique identifier, fabric switch identifier, and generated random number (operation 706). The switch checks how an identifier conflict should be resolved (operation 708). If the generated switch identifier does not require a conflict resolution, the switch associates the generated switch identifier with the fabric switch (operation 710). In some embodiments, associating the switch identifier with the fabric switch comprises associating the switch identifier with the local switch and notifying other member switches.

If the conflict should use user resolution (should be resolved by a user), the switch raises an exception to the user (e.g., an error message) and obtains a configured number from the user (operation 712). The switch then regenerates another switch identifier for the fabric switch based on the organizationally unique identifier, fabric switch identifier, and obtained configured number (operation 714). If the conflict should use automatic resolution (should be automatically resolved by the switch), the switch regenerates the random number (operation 722). The switch then continues to regenerate another switch identifier for the fabric switch based on the organizationally unique identifier, fabric switch identifier, and regenerated random number until the conflict is resolved (operation 724).

FIG. 7B presents a flowchart illustrating the process of a returning member switch of a fabric switch associating a unique identifier with the local switch, in accordance with an embodiment of the present invention. During operation, the switch joins a fabric switch as a member switch (operation 752) and retrieves the previous local configuration of the switch (operation 754). The switch then obtains the switch identifier associated with the fabric switch from the retrieved configuration (operation 756) and associates the local switch with the obtained switch identifier (operation 758).

Virtual Link Aggregation

An end device coupled to a fabric switch can be multi-homed (i.e., can be coupled to the fabric switch via multiple links, optionally with multiple member switches). When an end device is coupled to multiple member switches via multiple links, these links can be aggregated to one virtual link aggregation. Ports participating in a virtual link aggregation operate as a single port and should have a single port status. To address this issue, only one of the ports of the virtual link aggregation is allowed to participate in a spanning tree at a time. However, this may lead to inefficient port selection because another port can become a more suitable port at a later time.

FIG. 8A illustrates an exemplary virtual link aggregation with spanning tree support, in accordance with an embodiment of the present invention. A layer-2 network 800 includes fabric switch 802 and a switch 804. Fabric switch 802 includes member switches 812, 814, 816, and 818. Switch 804 is coupled to member switches 816 and 818 with virtual link aggregation 820. In this example, switch 804 can be considered an end device from fabric switch 802's perspective, and switches 816 and 818 are partner switches of virtual link aggregation 820. In some embodiments, fabric switch 802 can be a TRILL network and its member switches can be TRILL RBridges.

When switches 802 and 804 participate in a spanning tree, switch 804 views fabric switch 802 as a single switch. Switches 816 and 818 can be configured to operate in a special “trunked” mode for switch 804, where port 822 of switch 816 and port 824 of switch 818 operate as a single logical port of virtual link aggregation 820. As a result, the same port status should be associated with that logical port and have the same state transitions across a respective partner switch. This issue is addressed by allowing only one of the ports of virtual link aggregation 820 to actively participate in the spanning tree at a time. The switch that includes that port is referred to as the master switch. The edge port which receives the most recent proposal message is selected to participate in the spanning tree and the switch becomes the master switch. In other words, this selected port represents the logical port in the spanning tree.

Initially, the primary switch, which forwards multicast traffic via a virtual link aggregation, becomes the master switch. Suppose that switch 816 is the primary switch for virtual link aggregation 820. Hence, initially, switch 816 becomes the master switch; and port 822 participates in the spanning tree, sending and receiving BPDUs for the spanning tree. In other words, port 822 represents the logical port of virtual link aggregation 820 in the spanning tree. If switch 818 receives a BPDU via port 824, switch 818 becomes the master switch; and port 824 starts participating in the spanning tree, sending and receiving BPDUs for the spanning tree. In this way, BPDU is sent and received by the same node. In this way, no tunneling of BPDU to the primary switch is needed, and the transmit and receive state machines of the spanning tree protocol operate on the same member switch. Furthermore, if the current master switch of virtual link aggregation 820 becomes unavailable (e.g., due to a failure), another switch starts receiving BPDU and becomes the master switch.

FIG. 8B illustrates an exemplary virtual link aggregation with spanning tree support between fabric switches, in accordance with an embodiment of the present invention. In this example, in network 800, fabric switch 802 is coupled to fabric switch 806. Fabric switch 806 includes member switches 852, 854, and 856. Switch 814 is coupled to switch 852 via link 862, and switch 818 is coupled to switch 856 via link 864. Links 862 and 864 form a link aggregation 860 between fabric switches 802 and 806. In this case, both ends of virtual link aggregation are fabric switches. In this case, when fabric switches 802 and 806 are starting, both fabric switches 802 and 806 can receive BPDUs via non-primary switches. As a result, fabric switches 802 and 806 may chase each other, causing instability in network 800.

To solve this problem, the master switch is only changed in a fabric switch with an inferior fabric switch identifier (e.g., if the other fabric switch has a superior fabric switch identifier). Here, inferior or superior can be “greater than,” “less than,” or a combination thereof. For example, suppose that the fabric switch identifier of fabric switch 806 is inferior to the fabric switch identifier of fabric switch 802. Then the master switch is only changed in fabric switch 806. In this way, fabric switch 806 changes the master switch to match fabric switch 802. Suppose that the primary switch, and initial master switch, for virtual link aggregation 860 is switch 814 in fabric switch 802 and switch 856 in fabric switch 806. As a result, fabric switches 802 and 806 can both receive BPDUs via non-primary switches 818 and 852, respectively. However, only fabric switch 806 changes the master switch from primary switch 856 to switch 852. On the other hand, primary switch 814 remains the master switch in fabric switch 802. As a result, new master switch 852 of fabric switch 806 matches master switch 814 of fabric switch 802.

FIG. 9 illustrates an exemplary state diagram of a partner switch of a virtual link aggregation with spanning tree support, in accordance with an embodiment of the present invention. Initially, the switch is in an INIT state 902. If no primary switch is configured for the virtual link aggregation, the switch remains in the INIT state (transition 910). If the switch is a primary switch, the switch transitions to a MASTER state 904 (transition 912). In this state, the switch becomes the master switch for the virtual link aggregation and is responsible for actively participating in the spanning tree, sending and receiving BPDUs. In some embodiments, the switch maintains different state diagrams for different instances of the spanning tree protocol (e.g., different instances can have different master switches). If the switch is a non-primary switch, the switch transitions to a NON_MASTER state 906 (transition 914). In this state, the switch does not actively participate in the spanning tree. If the switch is the MASTER or NON_MASTER state, and spanning tree is disabled, the switch transitions back to the INIT state (transitions 916 and 918, respectively).

If the switch is in the NON_MASTER state and becomes a primary switch, the switch transitions to the MASTER state (transition 920). Similarly, if the switch is in the MASTER state and becomes a non-primary switch, the switch transitions to the NON_MASTER state (transition 922). If the switch is in the NON_MASTER state and receives a BPDU, the switch transitions to a REQ_MASTER state 908 (transition 924). In this state, the switch sends a request to become a master switch to other partner switches while remaining in the REQ_MASTER state (transition 926). If the switch is in the MASTER state and receives the request, the switch transitions to the NON_MASTER state (transition 928). Similarly, if the switch is in the REQ_MASTER state and receives a request to become a master switch from another partner switch, the switch transitions to the NON_MASTER state (transition 930). Transition 930 allows the partner switch most recently receiving a BPDU to become the master switch. If the switch is in the REQ_MASTER state and receives confirmations from all other partner switches, the switch transitions to the MASTER state (transition 932).

Exemplary Switch System

FIG. 10 illustrates an exemplary architecture of a switch with distributed spanning tree protocol support, in accordance with an embodiment of the present invention. In this example, a switch 1000 includes a number of communication ports 1002, a packet processor 1010, a spanning tree management module 1030, and a storage device 1050. Packet processor 1010 extracts and processes header information from the received frames.

In some embodiments, switch 1000 may maintain a membership in a fabric switch, as described in conjunction with FIG. 1A, wherein switch 1000 also includes a fabric switch management module 1060. In some embodiments, spanning tree management module 1030 represents the fabric switch as a single switch in a spanning tree. Fabric switch management module 1060 maintains a configuration database in storage device 1050 that maintains the configuration state of every switch within the fabric switch. Fabric switch management module 1060 maintains the state of the fabric switch, which is used to join other switches. In some embodiments, switch 1000 can be configured to operate in conjunction with a remote switch as an Ethernet switch. Under such a scenario, communication ports 1002 can include inter-switch communication channels for communication within a fabric switch. This inter-switch communication channel can be implemented via a regular communication port and based on any open or proprietary format. Communication ports 1002 can include one or more TRILL ports capable of receiving frames encapsulated in a TRILL header. Packet processor 1010 can process these TRILL-encapsulated frames.

During operation, packet processor 1010 obtains information associated with a spanning tree from a received message. Spanning tree management module 1030 checks whether the obtained information is superior to locally available information stored in storage device 1050 of the spanning tree, as described in conjunction with FIG. 2A. If so, spanning tree management module 1030 determines the port role of a local port, which is one of the communication ports of 1002, for the spanning tree to be the root port. Spanning tree management module 1030 also determines the port state of the local port for the spanning tree to be blocking. In some embodiments, switch 1000 includes a notification module 1032, which generates a notification message for a remote switch comprising the obtained information.

As described in conjunction with FIG. 2A, if switch 1000 receives an approval from a remote switch, spanning tree management module 1030 changes the port state of the local port for the spanning tree to be forwarding. On the other hand, if switch 1000 receives superior information of the spanning tree from a message, which is not a control message of the spanning tree, from a remote switch, spanning tree management module 1030 re-determines the port role of the local port for the spanning tree. Spanning tree management module 1030 stores the best locally available information of the spanning tree in a node root priority vector in storage device 1050 and the best information of the spanning tree associated with a remote switch in a local node root priority table in storage device 1050, as described in conjunction with FIGS. 1A and 1B. In some embodiments, the node root priority vector is stored in the node root priority table.

In some embodiments, switch 1000 also includes a port management module 1020, which assigns a fabric-switch-wide unique port identifier to the local port. Spanning tree management module 1030 uses this port identifier to participate in the spanning tree. Port management module 1020 stores the port identifier in an entry of a port identifier allocation table, which can be stored in storage device 1050. If a status update event occurs for switch 1000, port management module 1020 marks the entry as stale. When the effect of the status update event ends, port management module 1020 reassigns the port identifier to the local port, as described in conjunction with FIG. 4C.

If the number of port identifiers reaches its limit and the stale entry meets one or more reallocation criteria, port management module 1020 reassigns the port identifier of the stale entry to another port, as described in conjunction with FIG. 4D. Furthermore, if the number of port identifiers reaches its limit and no stale entry in the port identifier allocation table is available, port management module 1020 precludes switch 1000 from enabling the spanning tree for that other port. In some embodiments, fabric switch management module 1060 determines a switch identifier for the fabric switch. This switch identifier is distinct from the switch identifier of a second fabric switch, as described in conjunction with FIG. 6A.

In some embodiments, switch 1000 also includes a link aggregation module 1040, which operates the local port in conjunction with a second port of a remote switch as a single logical port of a virtual link aggregation. Link aggregation module 1040 also selects a master switch between switch 1000 and the remote switch. Link aggregation module 1040 can select the master switch based on whether a switch has received the most recent control message of the spanning tree, as described in conjunction with FIG. 8A. If switch 1000 is in a first fabric switch, and that first fabric switch is coupled to a second fabric switch, link aggregation module 1040 selects the master switch further based on whether a first identifier associated with the first fabric switch is inferior to a second identifier associated with the second fabric switch, as described in conjunction with FIG. 8B.

Note that the above-mentioned modules can be implemented in hardware as well as in software. In one embodiment, these modules can be embodied in computer-executable instructions stored in a memory which is coupled to one or more processors in switch 1000. When executed, these instructions cause the processor(s) to perform the aforementioned functions.

In summary, embodiments of the present invention provide a switch, a method and a system for facilitating external spanning tree support for a fabric switch. In one embodiment, the switch includes a packet processor and a spanning tree management module. The packet processor obtains information associated with a spanning tree from a message. The spanning tree management module, in response to the obtained information being superior to locally available information of the spanning tree, determines the port role of a local port of the switch for the spanning tree to be the root port and the port state of the local port for the spanning tree to be blocking.

The methods and processes described herein can be embodied as code and/or data, which can be stored in a computer-readable non-transitory storage medium. When a computer system reads and executes the code and/or data stored on the computer-readable non-transitory storage medium, the computer system performs the methods and processes embodied as data structures and code and stored within the medium.

The methods and processes described herein can be executed by and/or included in hardware modules or apparatus. These modules or apparatus may include, but are not limited to, an application-specific integrated circuit (ASIC) chip, a field-programmable gate array (FPGA), a dedicated or shared processor that executes a particular software module or a piece of code at a particular time, and/or other programmable-logic devices now known or later developed. When the hardware modules or apparatus are activated, they perform the methods and processes included within them.

The foregoing descriptions of embodiments of the present invention have been presented only for purposes of illustration and description. They are not intended to be exhaustive or to limit this disclosure. Accordingly, many modifications and variations will be apparent to practitioners skilled in the art. The scope of the present invention is defined by the appended claims. 

What is claimed is:
 1. A switch, comprising: a packet processor adapted to obtain information associated with a spanning tree from a message; and a spanning tree management module adapted to, in response to the obtained information being superior to locally available information of the spanning tree: determine a port role of a local port of the switch for the spanning tree to be root port; and determine a port state of the local port for the spanning tree to be blocking.
 2. The switch of claim 1, further comprising a notification module adapted to generate a notification message for a remote switch comprising the obtained information.
 3. The switch of claim 1, wherein the spanning tree management module is further adapted to change the port state of the local port for the spanning tree to be forwarding in response to an approval from a remote switch.
 4. The switch of claim 1, wherein the spanning tree management module is further adapted to re-determine the port role of the local port for the spanning tree in response to obtaining superior information of the spanning tree from a message from a remote switch, wherein the message is not a control message of the spanning tree.
 5. The switch of claim 1, wherein the spanning tree management module is further adapted to: store best locally available information of the spanning tree in a local node root priority vector; and store best information of the spanning tree associated with a remote switch in a local node root priority table.
 6. The switch of claim 1, further comprising a fabric switch management module adapted to maintain a membership in a fabric switch, wherein the fabric switch is configured to accommodate a plurality of switches and operates as a single switch; and wherein the spanning tree management module is further adapted to represent the fabric switch as a single switch in the spanning tree.
 7. The switch of claim 6, further comprising a port management module adapted to assign a port identifier to the local port, wherein the port identifier is unique among edge ports of the fabric switch; and wherein the spanning tree management module is further adapted to use the port identifier to participate in the spanning tree, thereby facilitating the local port to appear as a port of the single switch represented by the fabric switch.
 8. The switch of claim 7, wherein the port management module is further adapted to: store the port identifier in an entry of a port identifier allocation table of the switch; and mark the entry as stale in response to a status update event occurring for the switch; and wherein other member switches of the fabric switch are precluded from assigning the stale entry to an edge port.
 9. The switch of claim 8, wherein the port management module is further adapted to reassign the port identifier to the local port in response to an ending of an effect of the status update event.
 10. The switch of claim 8, wherein the port management module is further adapted to reassign the port identifier of the stale entry to a second port in response to reaching a limit of a number of port identifiers and the stale entry meeting one or more reallocation criteria.
 11. The switch of claim 10, wherein the reallocation criteria include: an age of a stale entry; and an indication whether an entry is stale due to a configured event or a learned event.
 12. The switch of claim 8, wherein the port management module is further adapted to preclude the switch from enabling spanning tree for a local port in response to reaching a limit of a number of port identifiers and an unavailability of a stale entry in the port identifier allocation table.
 13. The switch of claim 6, wherein the fabric switch management module is further adapted to determine a first switch identifier for the fabric switch, wherein the first switch identifier is distinct from a second switch identifier associated with a second fabric switch, and wherein the distinction between the first and the second switch identifiers is based on a random number or a configured number.
 14. The switch of claim 1, further comprising a link aggregation module adapted to: operate the local port in conjunction with a second port of a remote switch as a single logical port of a virtual link aggregation; and select a master switch between the switch and the remote switch, wherein the master switch includes a selected port between the local port and the second port, and wherein the selected port actively participates in the spanning tree as representative of the logical port.
 15. The switch of claim 14, wherein the link aggregation module selects the master switch based on whether a switch has received the most recent control message of the spanning tree.
 16. The switch of claim 15, wherein the link aggregation module selects the master switch further based on whether a first identifier associated with the switch and the remote switch is inferior to a second identifier, wherein the first identifier is associated with a first fabric switch and the second identifier is associated with a second fabric switch, and wherein a fabric switch is configured to accommodate a plurality of switches and operates as a single switch.
 17. A computer-executed method, comprising: obtaining information associated with a spanning tree from a message; and in response to the obtained information being superior to locally available information of the spanning tree in a switch: determining a port role of a local port of the switch for the spanning tree to be root port; and determining a port state of the local port for the spanning tree to be blocking.
 18. The method of claim 17, further comprising generating a notification message for a remote switch comprising the obtained information.
 19. The method of claim 17, further comprising changing the port state of the local port for the spanning tree to be forwarding in response to an approval from a remote switch.
 20. The method of claim 17, further comprising re-determining the port role of the local port for the spanning tree in response to obtaining superior information of the spanning tree from a message from a remote switch, wherein the message is not a control message of the spanning tree.
 21. The method of claim 17, further comprising: storing best locally available information of the spanning tree in a local node root priority vector; and store best information of the spanning tree associated with a remote switch in a local node root priority table.
 22. The method of claim 17, further comprising: maintaining a membership in a fabric switch, wherein the fabric switch is configured to accommodate a plurality of switches and operates as a single switch; and representing the fabric switch as a single switch in the spanning tree.
 23. The method of claim 22, further comprising: assigning a port identifier to the local port, wherein the port identifier is unique among edge ports of the fabric switch; and using the port identifier to participate in the spanning tree, thereby facilitating the local port to appear as a port of the single switch represented by the fabric switch.
 24. The method of claim 23, further comprising: storing the port identifier in an entry of a port identifier allocation table of the switch; and marking the entry as stale in response to a status update event occurring for the switch; wherein other member switches of the fabric switch are precluded from assigning the stale entry to an edge port.
 25. The method of claim 24, further comprising reassigning the port identifier to the local port in response to an ending of an effect of the status update event.
 26. The method of claim 24, further comprising reassigning the port identifier of the stale entry to a second port in response to reaching a limit of a number of port identifiers and the stale entry meeting one or more reallocation criteria.
 27. The method of claim 26, wherein the reallocation criteria include: an age of a stale entry; and an indication whether an entry is stale due to a configured event or a learned event.
 28. The method of claim 24, further comprising precluding the switch from enabling spanning tree for a local port in response to reaching a limit of a number of port identifiers and an unavailability of a stale entry in the port identifier allocation table.
 29. The method of claim 22, further comprising determining a first switch identifier for the fabric switch, wherein the first switch identifier is distinct from a second switch identifier associated with a second fabric switch, and wherein the distinction between the first and the second switch identifiers is based on a random number or a configured number.
 30. The method of claim 17, further comprising: operating the local port in conjunction with a second port of a remote switch as a single logical port of a virtual link aggregation; and selecting a master switch between the switch and the remote switch, wherein the master switch includes a selected port between the local port and the second port, and wherein the selected port actively participates in the spanning tree as representative of the logical port.
 31. The method of claim 30, wherein the master switch is selected based on whether a switch has received the most recent control message of the spanning tree.
 32. The method of claim 31, wherein the master switch is selected further based on whether a first identifier associated with the switch and the remote switch is inferior to a second identifier, wherein the first identifier is associated with a first fabric switch and the second identifier is associated with a second fabric switch, and wherein a fabric switch is configured to accommodate a plurality of switches and operates as a single switch.
 33. A computer system, comprising: a processor; and a storage device coupled to the processor and storing instructions which when executed by the processor cause the processor to perform a method, the method comprising: obtaining information associated with a spanning tree from a message; and in response to the obtained information being superior to locally available information of the spanning tree in a switch: determining a port role of a local port of the switch for the spanning tree to be root port; and determining a port state of the local port for the spanning tree to be blocking. 